This invention relates to a fuel assembly for a reactor. The fuel assembly consists of a predetermined number of fuel rods that are bundled by a support grating i.e., a channel box, in a group as a handling unit. A predetermined number of fuel assemblies are set up in moderators in the reactor and the gaps between the fuel assemblies are kept so that the fuel assemblies can be individually inserted or withdrawn or control rods can be easily inserted between the fuel assemblies. Accordingly, the fuel rods located on the periphery of the fuel assemblies are surrounded by a greater number of moderators than those located at the central portion. As a result, thermalization of neutrons proceeds effectively on the periphery of fuel assemblies to cause more vigorous nuclear fission, to raise the local output peaking coefficient on the periphery and to reduce thermal allowance. For these reasons, the enrichment gradation (lowering the fuel pellets enrichment on the periphery of the assembly than at the central portion) has been employed in the conventional boiling water reactors in order to equalize the local output peaking factors of the fuel assemblies. However, this method invites the following problems.
(1) The fissionable material is left unburnt at the central portion of the fuel assembly and hence, the fuel resources can not be utilized effectively.
(2) Since the fuel having higher enrichment must be disposed at the central portion of fuel assembly where having smaller importance while the fuel having lower enrichment must be disposed on the periphery of the fuel assembly where having greater importance, the infinite neutron multiplication factor of the fuel assemblies as a mean value drops.
(3) Fuel pellets having higher enrichment than the mean enrichment must be used in order to give the enrichment gradation. This reduces the maximal value of the atomic number of the fissionable material to be packed into the fuel assembly since there exists a limitation to the highest pellet enrichment at present, and reduces the derivable discharged burnup. This is not desirable for a high burnup core using high enrichment fuel pellets.